Pomatrum ventralis (synonym Xidazoon stephanus)
Pomatrum ventralis (Xidazoon stephanus) is a marine animal found from the Chengjiang Lagerstätte, the Lower Cambrian 4). It is classified into Family Diadazoon, Order Vetulicolita, and Class Vetulicolida 1), but its upper phylogeny is still problematic. Fossils within Class Vetulicolida have been found from North America, Australia 1), 2), 5).
The name Pomatrum and Xidazoon were published in same year. They are considered as a same species, and Xidazoon would be a synonym of Pomatrum. But some argue that their shapes of posterior halves are different, at present, this question do not come to conclusion 1).
The body of Pomatrum, consists of two parts, anterior half and posterior half, and these parts are connected by a constriction. The anterior body is about 50 mm in length, about 25-30 mm in height, and the posterior body is approximately similar length to the anterior 1).
The anterior body is nearly ovoid or rounded rectangle in lateral view, with six segments, an array of lateral pouches presuming gills or gill-slits, and an oral opening surrounded by inner and outer oral discs marked radiating lines 1).
The posterior body has multiple segments, but the boundaries are unclear 1).
There is an alimentary tract in the posterior body, which joins the anterior body cavity filled with sediments 1).
It does not rule out the possibility that Pomatrum could swam in the middle of the water column, but the guts and the body cavity had filled with sediments, so they may have been swimming near the bottom of the sea and feeding from the sediment surface 1).
Vetulicolians had a lateral groove with a series of gill-slits, and they may have a pumping mechanism that could process considerable volumes of seawater 3).
If that is the case, they may had used not only for gas change, but also for feeding and for water-jet propulsion. Unexpectedly, they may have been a good swimmer.
The morphology of Pomatrum make us imagine a free-swimming tadpole-shaped larval tunicate (ascidians or sea squirts). Although it still remains some discussions, the vetulicolians are interpreted as stem-group deuterostomes 3), 5), and are considered to be close to the tunicates 1), or a sister-group to the tunicates 2).
- Aldridge RJ, Xian‐Guang H, Sivester DJ, Sivester DJ, Gabbott SE (2007) The systematics and phylogenetix relationships of Vetulicolians. Palaeontology 50: 131-168. (DOI:10.1111/j.1475-4983.2006.00606.x.)
- García-Bellido DC, Lee MSY, Edgecombe GD, Jago JB, Gehling JG, Paterson JR (2014) A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC Evol Biol 14, 214. (DOI:10.1186/s12862-014-0214-z.)
- Ou Q, Conway Morris S, Han J, Zhang Z, Liu J, Chen A, Zhang X, Shu D (2012) Evidence for gill slits and a pharynx in Cambrian vetulicolians: implications for the early evolution of deuterostomes. BMC Biol 10, 81. (DOI: 10.1186/1741-7007-10-81.)
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- Vinther J, Smith MP, Harper DAT (2011) Vetulicolians from the Lower Cambrian Sirius Passet Lagerstätte, North Greenland, and the polarity of morphological characters in basal deuterostomes. Palaeontology 54: 711-719. (DOI:10.1111/j.1475-4983.2011.01034.x.)